STABILITY ANALYSIS OF POLYNOMIALS WITH COEFFICIENTS IN DISKS

The aim of this note is to report results on the stability of a class of polynomials from the small gain theorem point of view. We consider families of polynomials whose coefficients lie in closed circular disks around their nominal values. Various measures of variation of polynomial coefficients around their nominal value are considered and in each case necessary and sufficient conditions are presented for stability of the resulting family of polynomials. The stability region could be any closed region of the complex plane. Based on similar ideas of small gain, we also provide sufficient conditions for testing the stability of systems with commensurate time delays, and for two-dimensional type systems. These conditions become both necessary and sufficient in some special cases. All tests are easy to implement and require checking the stability of a matrix (or equivalently checking the stability of the ''central'' polynomial) and evaluation of a norm.